TECHNIQUE FOR THE DRY TRANSFER OF EPITAXIAL GRAPHENE ONTO ARBITRARY SUBSTRATES

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1 TECHNIQUE FOR THE RY TRANSFER OF EPITAXIAL RAPHENE ONTO ARBITRARY SUBSTRATES Joshua. Caldwell, 1 Travis J. Anderson, 1 James C. Culbertson, 1 lenn. Jernigan, 1 Karl. Hobart, 1 Fritz J. Kub, 1 Marko J. Tadjer, 2 Joseph L. Tedesco, 1 Jennifer K. Hite, 1 Michael A. Mastro, 1 Rachael L. Myers-Ward, 1 Charles R. Eddy Jr., 1 Paul M. Campbell 1 and. Kurt askill 1 1 U.S. Naval Research Laboratory, 4555 Overlook Ave, S.W., Washington,.C Electrical Engineering epartment, University of Maryland, College Park, M Joshua.caldwell@nrl.navy.mil RECEIVE ATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) Abstractn order to make graphene technologically viable, the transfer of graphene films to substrates appropriate for specific applications is required. We demonstrate the dry transfer of epitaxial graphene (E) from the C-face of 4H-SiC onto SiO 2, an and Al 2 O 3 substrates using a thermal release tape. We further report on the impact of this process on the electrical properties of the E films. This process enables E films to be used in flexible electronic devices or as optically transparent contacts. Reports of single layer graphene have drawn significant interest due to its exciting properties, such as ballistic carrier transport, 1, 2 high thermal and electrical conductivity, optical transmission, 3-5 and high mechanical hardness. 6 Films used in those studies were created primarily via the exfoliation method, 1, 5 which produces small-area flakes with variable size, shape and thickness. In order to use graphene as a conductive, optically transparent contact, the reproducible transfer of large-area graphene films that could subsequently be patterned into top-side contacts is required. Large-area graphene films can be produced by either metal-catalyzed growth on films of nickel 7-9 or copper 10 or by epitaxial growth through the sublimation of silicon from the surface of silicon carbide (SiC) Epitaxial graphene (E) films grown on the carbon-terminated surface of SiC have been shown to be of high quality, with room temperature Hall mobilities up to 4,200 cm 2 /Vs having been observed in 16 mm x 16 mm C-face E films grown within our lab. 17 Furthermore, we have observed room temperature Hall effect mobilities as high as 23,000 cm 2 /Vs from specific 10 μm Hall crosses fabricated on C-face grown E films. However, the C-face films, while exhibiting significantly higher mobilities than E films grown on the Si-face, do tend to have a high variability in the E film thickness across a given sample. Here we present results illustrating a dry transfer technique using Nitto enko Revalpha thermal release tape that has enabled the transfer of large areas (squares up to 16 mm on a side) of E from the C-face SiC donor substrate onto SiO 2 on Si handle substrates. For this study C-face grown E material was chosen due to its high carrier mobility. Successful transfers of C-face E onto an and Al 2 O 3 films will also be discussed, demonstrating that this process can be readily implemented for use with other handle substrates of interest. To date, the only reported transfer of E from SiC utilized the exfoliation method, 18 producing sub-micron-sized samples, which are not amenable to large-scale device manufacturing. While a wet chemical approach has been successfully used to transfer metal- 1

2 catalyzed graphene, 7-9 this process is not amenable to E on SiC, as SiC is highly resistant to chemical etchants. We further report on Hall effect measurements that were performed on both the large-area transferred E films and fabricated van der Pauw devices to determine the influence of the transfer process and subsequent lithography on the electrical properties of the transferred E films. Raman spectroscopy was used to verify the quality and thickness of the E films and X-ray photoemission spectroscopy (XPS) measurements were performed to verify the transfer efficiency. In addition, the XPS characterization of E films during the transfer process revealed the presence of atomic silicon. These measurements all show that this transfer process can create large-area E films on arbitrary substrates suitable for both device fabrication and further experiments exploring the impact of the substrate electrical properties upon the electrical behavior of the E films. E was grown for these transfer experiments via the sublimation process on 2 C-face 4H-SiC 15, 16 substrates that were previously chemically-mechanically polished. The substrates were placed within an Aixtron/Epigress VP508 Hot-Wall chemical vapor deposition reactor and the surface was further prepared via an H 2 etch at 100 mbar and 1600 o C for 20 min. The chamber was evacuated to (1.4-17)x10-4 mbar and the temperature was lowered to 1550 o C for E formation. E growth was carried out for 1 hour. After growth, the chamber was allowed to cool overnight. 16 Square samples 10 mm on a side were cut from the 2 C-face 4H-SiC substrate following E growth, to enable multiple transfer attempts. The SiO 2 on Si handle substrates used for most E transfers consisted of 100 nm of thermally-grown SiO 2 on n-type Si. In order to improve the strength of bonding between the transferred E and the SiO 2 surface, a cleaning and surface preparation procedure was used to produce a hydrophilic surface on the SiO 2. The SiO 2 surface was cleaned using a 750 W O 2 plasma treatment for 5 min in a Plasma-Preen II-973, followed by ultrasonic SC1 cleaning (5:1:1, H 2 O:NH 4 OH:H 2 O 2 ) at 40 o C for 14 min, ending with a 1 min rinse in megasonic water provided by a Honda Electronics PulseJet (W-357-3MP) system at full power. Immediately before the transfer, the handle substrates were treated with a 30 s, 750 W O 2 plasma treatment, a 1 min ultrasonic SC1 clean and a 1 min megasonic rinse to ensure a hydrophilic SiO 2 surface was produced. With the exception of a standard solvent clean, the E surface did not require any specific preparation procedures for a successful transfer. For the E transfers to p- and n- type an, 2 μm thick an films were grown on 2-inch, a-plane sapphire substrates using metal organic chemical vapor deposition (MOCV). The deposition was initiated with a 25 nm AlN nucleation layer grown at 680 o C and 50 Torr with the subsequent an growth deposited at 1025 o C and 50 Torr. The n- and p-type doping was accomplished with disilane and Cp 2 Mg, respectively. E films were also transferred onto 100 nm Al 2 O 3 films grown via atomic layer deposition (AL) on a 20 mm x 20 mm double-side polished, epi-grade, c-face sapphire substrate at a chamber temperature of 300 o C using tetramethyl aluminum and water as the alternating precursors. For the E transfer to both an and Al 2 O 3, the surface preparation of the handle substrate consisted of only a 5 min, 750 W O 2 plasma treatment in the Plasma Preen system. Following the prescribed substrate pretreatment procedures, a precut piece of Nitto enko Revalpha Thermal Release Tape (Part No. 3193MS, 7.3 N/mm) was placed on the E surface. The tape/e/sic sample stack was then placed on a silicon wafer within the bore of an EV EV501 wafer bonding apparatus. A 2 stainless steel pressure plate used for applying a uniform force to the stack was then placed on top of the stack. Following an evacuation of the bonding chamber to a pressure of approximately 5x10-4 Torr, a force between 3-6 N/mm 2 was applied to the stack for 10 min. After this process, the sample stack was removed from the bonder and the tape was peeled from the SiC wafer, thereby removing approximately 90% of the E layers from the SiC as measured by both Raman and XPS. The tape with the removed E layers was placed on the prepared SiO 2 on Si handle substrate and was then returned to the bonding apparatus, underneath the 2 stainless steel pressure plate, where a force of 3-6 N/mm 2 was applied for 10 min. The stack was then removed and placed on a hot plate, where the surface temperature was stabilized at a temperature 1-2 o C above the 120 o C release temperature of the tape. This thermal treatment eliminates the adhesion strength of the tape. The tape was removed, leaving behind the transferred E film on the handle substrate. The tape residue was dissolved using a solution of 1:1:1 toluene:methanol:acetone and a final anneal at 250 o C for 10 min was 2

3 performed to improve the transferred E to SiO 2 adhesion. A schematic of this process is depicted in the Supplemental Information section. E film thicknesses were estimated by measuring the attenuation of substrate Raman signal intensity (777 and 964 cm -1 for E on SiC; 521 cm -1 for E transferred to SiO 2 on Si) induced by the presence of the E film. 19 Atomic force microscopy of various regions of interest was used to calibrate the attenuated Raman signal intensity to the measured E film thickness, fitting the results to the following: 2 I = I0e αd (1) where I and I0 represent the measured Raman intensity in the presence and absence of an E film, respectively. The fitting parameter α corresponds to the relative extinction coefficient of the E films on SiC, (α =0.2 nm -1 ) and of the E and SiO 2 films on Si, (α = nm -1 ). Raman measurements were performed confocally using either a or a 532 nm laser line focused through a 100X, 0.9 N.A. or a 50X, 0.42 N.A. objective, respectively, in both cases providing a sub-micron laser spot with a power at the sample of approximately 10 mw. The collected signal was passed through an appropriate wavelength Semrock long-pass filter and was detected using either an Ocean Optics QE65000 CC spectrometer or a half-meter Acton single spectrometer (SpectraPro-2500i) with a Princeton Instruments nitrogen-cooled, back-thinned, deep-depleted CC array detector (SPEC-10:400BR/LN). Spatial mapping of the Raman peak characteristics (position, full-width at half max, center-of-mass, intensity and calculated E film thickness) was achieved by translating the sample with respect to the laser spot. In order to determine the effect of the transfer process on the electrical characteristics of the transferred E films, Hall effect mobility and carrier density measurements were carried out at 300 K using a van der Pauw configuration. For 10 mm x 10 mm samples, beryllium-copper pressure clips were used as probe contacts to the corners of the as-grown and transferred films. For patterned 200 μm x 200 μm van der Pauw squares, standard probe manipulators were used as current and voltage leads. Measurement currents ranged from 1 to 100 μa while the magnetic field was approximately 2k. Presented in Fig. 1 are two-dimensional Raman thickness maps, using the characterization technique described above, for an (a) as-grown E film on C-face 4H-SiC, (b) an E film transferred using 3 N/mm 2 bonding force onto a SiO 2 on Si substrate and (c) a residual E film on C-face 4H-SiC following the successful removal of the upper E layers. The map of the as-grown E film [Fig. 1 (a)], indicates that there is a some considerable variation in the initial film thickness, with the average thickness of this sample found to be approximately 19 nm, with E film thicknesses typically ranging from nm. In comparison, the transferred E films had average thicknesses ranging from 8-14 nm. Similar variations in the average film thickness of the as-grown and transferred films were observed. ue to these comparable thickness variations observed before and after the E transfer process, it was difficult to determine if any changes in the thickness uniformity were induced via the transfer. We also determined that the thickness of the residual E layer on SiC following the transfer ranged from 0-5 nm in thickness, implying that about 87% of the E film was removed via the transfer procedure. Subsequent XPS measurements verified this transfer efficiency. Figure 1: Spatial maps of the film thickness for (a) an as-grown E film on SiC, (b) an E film transferred using a 3 N/mm 2 bonding force onto a SiO 2 on Si handle substrate and (c) the residual E film remaining on SiC after the transfer process. The film thickness was measured using the method outlined in Ref. 19 and calibrated using AFM. In Fig. 1(b), holes on the order of 5-10 μm in size can be observed in the transferred E films, which are not present in the as-grown films as illustrated in Fig. 1 (a). In Fig. 1 (c), similar sized regions of thick E were found remaining on the SiC surface after the transfer process. Presumably, these regions of missing E within the transferred films correlate to the thick residual E regions on the SiC after the transfer. In the as-grown E films, small regions with nm reductions in E film thickness can also be observed. It is possible that the small holes in the transferred films are due to the inability of the bonding force to fully press the transfer 3

4 tape into these regions where steep reductions in film thickness are found, thus leaving the observed void in the transferred E films. In order to optimize the transfer process, a bonding force dependence study was performed. The force applied to the tape/e/sic stack was varied from 3-6 N/mm 2, with a constant 3 N/mm 2 force applied during the second bonding step Figure 2: Nomarski micrographs of E films transferred to SiO 2 on Si substrates following the application of (a) 3, (b) 4 and (c) 5 N/mm 2 force during the bonding of the thermal release tape to the E on SiC prior to removal. Regions of missing E have been highlighted in red for clarity and the percentage of the field-of-view where graphene is missing is reported in the lower left hand corner of the images. (tape/e/handle substrate stack). Nomarski micrographs of the films transferred using the 3, 4 and 5 N/mm 2 bonding forces were collected through a 10X, 0.25 NA objective and are presented in Fig. 2 (a)-(c), respectively. These images illustrate that as the force is increased from 3 to 5 N/mm 2, the number of regions of missing E (highlighted in red) in the transferred material are reduced. Therefore increasing the force during the tape to E bonding stage leads to a more complete and continuous E film. Subsequent particle analysis studies were performed to measure the percentage of the microscope field-of-view where E was missing and the corresponding values are presented in Fig. 2 (a)-(c). This was completed using the analyze particles function of ImageJ. 20 These measurements further illustrate the improvement in the transferred E film quality with increasing bonding force, as the areal percentage of missing E was reduced ten-fold as the pressure was increased from 3 to 5 N/mm 2. Presented in Fig. 3 (a) and (b) are μ-raman spectra collected from the E films transferred using 3 and 5 N/mm 2 bonding force, respectively. These spectra depict the relative intensities of the (~1380 cm -1 ), (~1530 cm -1 ) and 2 (~2700 cm -1 ) Raman lines from selected areas with the highest (red trace) and lowest (blue trace) : intensity ratios ( : ) I I. Previously, it was reported that this ratio varies inversely with the planar correlation length (approximate grain size) of the graphitic planes and may be used as a qualitative figure of merit for E films, with a low I indicating a higher quality graphene film. 21 Corresponding spatial maps of the I ratio over a 20 μm x 20 μm area from these E films. A significant reduction in the I ratio was observed in films transferred using the higher bonding force, thereby indicating a dramatic improvement in the material quality with increasing bonding force. Furthermore, the spatial uniformity of the I ratio was significantly increased in the film transferred using a force of 5 N/mm 2. The average values of the I ratio were / and / for the films transferred using 3 and 5 N/mm 2, respectively, with the error reported pertaining to the 95% confidence interval. By comparison, I ratios of the as-grown E films from the same wafer were found to range from to to 0.015, indicating that the optimized transfer process induced minimal additional degradation to the E films. Upon increasing the pressure to 6 N/mm 2 (not shown), the uniformity of the transferred film was lost, as small islands of transferred E were observed instead of a continuous E film. This force dependence study indicates that a bonding force of approximately 5 N/mm 2 is optimal for enabling the transfer of the most continuous, uniform and lowest defectivity E films from SiC using this thermal release tape. However, efforts involving graphene transfers from different growth substrates and/or different adhesives would require a separate optimization study. In subsequent experiments it was also determined that increasing the bonding force to the tape/e/handle substrate stack to 5 N/mm 2 also improved the efficiency of the transfer, as illustrated by a further improved transfer efficiency of the E films onto the handle substrates. To determine the amenability of this transfer process to various other handle substrates, the optimized process described above, with the exception of the handle substrate surface preparation steps, was used 4

5 to transfer E films from C-face SiC onto p- and n-type an and also onto Al 2 O 3 films deposited on c- face sapphire via AL. In the case of the former, the AL Al 2 O 3 film was required to modify the highly unreactive surface of the sapphire substrate to enable a successful transfer. The transferred E films optically appeared very similar to those transferred onto SiO 2 using the same bonding force, therefore indicating that the transfer process is relatively insensitive to the handle substrate material. In order to determine if any degradation to the electrical properties of the transferred films occurred during the transfer process, Hall measurements were initially performed on asgrown E films and were repeated following the transfer of these films to SiO 2. Room temperature carrier mobilities from the 16 mm x 16 mm asgrown E material ranged from 909 to 1875 cm 2 /Vs, with an average mobility of 1485 cm 2 /Vs being observed, corresponding with an average carrier density of 15.8 x cm -3. Following the transfer process, the mobility and carrier density from these large-area films both decreased Figure 3: μ-raman spectra collected from E films transferred to SiO2 on Si substrates using a bonding force of (a) 3 and (b) 5 N/mm 2. The spectra were collected from regions where there was high (red trace) and low (blue trace) intensities of the Raman line. Corresponding maps representing the spatial distribution of the ratio of the Raman to intensities are presented in (c) and (d), respectively. significantly, with post-transfer mobility values ranging from 188 to 269 cm 2 /Vs, with an average of 201 cm 2 /Vs, while the carrier density was reduced three-fold to an average of 5.10 x cm - 2. Because this reduction in mobility was observed despite a corresponding decrease in carrier density, it is apparent that this mobility reduction is due to the introduction of additional scattering centers or defects within the E films during the transfer process. However, even this reduced mobility is still orders of magnitude higher than amorphous silicon or most flexible conductive films, such as organic thin film transistors. 22 A large reduction in the carrier density in comparison to the values initially measured in the as-grown E films was observed via the Hall effect following the transfer process. Sun et al. 23 showed that within a C-face E film that there are layers with varied levels of doping and they further proposed that the highest doped layers are located at or near the SiC-E interface. As the transfer procedure discussed here leaves behind only the bottom most layers, the reduction in carrier density observed in the transferred E films is consistent with the upper E layers having fewer carriers than those layers closest to the SiC interface. One would further expect that the residual E films on SiC would exhibit a carrier density that was unchanged from the initial as-grown E film measurements. However, due to a lack of electrical continuity of the post-transfer, residual E films, Hall measurements were inconclusive. XPS measurements of E films bound to the thermal-release tape prior to transfer, revealed the presence of a Si 2p peak within the E films. Subsequent measurements illustrated that the tape was not the source of this Si peak. Therefore, it is possible that these intercalated Si atoms are acting as an isoelectronic dopant and may account, at least in part, for the high sheet carrier densities 24, 25 found in E in comparison to the reported intrinsic levels. Previous reports 16 have illustrated that fabrication of smaller area devices ( μm on a side) within E material has enabled the observation of significantly higher carrier mobilities. In an effort to determine if such increases in carrier mobility would be observed within the transferred E films and to determine the suitability of these films for electrical device fabrication, one of the films transferred using a force of 3 N/mm 2 was patterned into a series of device structures including multiple 200 μm x 200 μm van der Pauw squares with gold contact pads. Presented in Fig. 4 (a) is an optical micrograph of one such device. Note the small region in the upper left where E is not present. Subsequent Raman spatial thickness maps [Fig. 4 (b)] were performed and highlight the locations of the missing E (shown as black in the spatial map) and provide a measured average E film thickness of approximately 10 nm. 5

6 Figure 4: (a) Optical image of an E film patterned into a 200 μm x 200 μm van der Pauw square and (b) the corresponding thickness map of this patterned device determined using the Raman technique outlined in the text. espite these van der Pauw squares typically being incomplete films, the mobilities were still found to improve relative to the large-area transferred films, with values ranging from 384 to 803 cm 2 /Vs, with an average of 739 cm 2 /Vs, being recorded. These values were therefore approaching 50% of the initial mobilities measured in the as-grown E films. Here we have outlined a method enabling the dry transfer of large-area C-face E films from SiC onto an arbitrary handle substrate, thereby greatly increasing the flexibility of graphene films for most electronic, optoelectronic and mechanical applications. While we have focused this discussion on the transfer of E onto SiO 2 on Si substrates, successful transfers onto both p- and n-type MOCV an and thin AL-deposited Al 2 O 3 films were also reported. Using this process, patterned devices were fabricated that retained up to 50% of the asgrown carrier mobilities, while a large reduction in the carrier density and the presence of Si within the E films was observed. These transferred E films are suitable for optically transparent, conductive films for optoelectronic applications and the observed mobilities are several orders of magnitude higher than either amorphous silicon or most flexible electronic devices, such as organic thin film transistors. 22 Acknowledgements: We would like to thank Nitto enko America (Fremont, CA), Electronics Process Material roup (system.nda@nitto.com) for providing the thermal release tape and expertise in the associated transfer and cleaning processes. The authors also express their appreciation to Mr. Steven Binari for the use of his Hall effect measurement system and r. Jeremy Robinson for helpful discussions. This work was supported in part by the Office of Naval Research. Support for JLT and JKH was provided by the ASEE. REFERENCES 1. Novoselov, K. S.; eim, A. K.; Morozov, S. V.; Jiang,.; Zhang, Y.; ubonos, S. V.; rigorieva, I. V.; Firsov, A. A., Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, (5696), Avouris, P.; Chen, Z.; Perebeinos, V., Carbon-based electronics. Nature Nanotech. 2007, 2, Kuzmenko, A. B.; van Heumen, E.; Carbone, F.; van der Marel,., Universal infrared conductance of graphite. Phys. Rev. Lett. 2008, 100, (11), Nair, R. R.; Blake, P.; rigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M. R.; eim, A. K., Fine Structure Constant efines Visual Transparency of raphene. Science 2008, 320, Wang, X.; Linjie, Z.; Mullen, K., Transparent, Conductive raphene Electrodes for ye- Sensitized Solar Cells. Nano Lett. 2008, 8, (1), Lee, C.; Wei, X.; Kysar, J. W.; Hone, J., Measurement of the Elastic Properties and Intrinsic Strength of Monolayer raphene. Science 2008, 321, (5887), Yu, Q. K.; Lian, J.; Siriponglert, S.; Li, H.; Chen, Y. P.; Pei, S. S., raphene segregated on Ni surfaces and transferred to insulators. Appl. Phys. Lett. 2008, 93, (11), Reina, A.; Jia, X. T.; Ho, J.; Nezich,.; Son, H. B.; Bulovic, V.; resselhaus, M. S.; Kong, J., Large Area, Few-Layer raphene Films on Arbitrary Substrates by Chemical Vapor eposition. Nano Lett. 2009, 9, (1),

7 9. Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, P.; Choi, J. Y.; Hong, B. H., Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature 2009, 457, (7230), Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang,.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S., Large-Area Synthesis of High-Quality and Uniform raphene Films on Copper Foils. Science Express 2009, in press. 11. Berger, C.; Song, Z. M.; Li, T. B.; Li, X. B.; Ogbazghi, A. Y.; Feng, R.; ai, Z. T.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A., Ultrathin epitaxial graphite: 2 electron gas properties and a route toward graphene-based nanoelectronics. J. Phys. Chem. B 2004, 108, (52), Forbeaux, I.; Themlin, J. M.; ebever, J. M., Heteroepitaxial graphite on 6H-SiC(0001): Interface formation through conduction-band electronic structure. Phys. Rev. B: Condens. Matter 1998, 58, (24), Charrier, A.; Coati, A.; Argunova, T., Solid-state decomposition of silicon carbide for growing ultra-thin heteroepitaxial graphite films. J. Appl. Phys. 2002, 92, (2479). 14. askill,. K.; Jernigan,..; Campbell, P. M.; Tedesco, J. L.; Culbertson, J. C.; vanmil, B. L.; Myers-Ward, R. L.; Eddy, C. R.; Moon, J.; Curtis,.; Hu, M.; Wong,.; Mcuire, C.; Robinson, J. A.; Fanton, M. A.; Stitt, J. P.; Stitt, T.; Snyder,.; Frantz, E., ECS Trans. 2009, 19, Jernigan,..; vanmil, B. L.; Tedesco, J. L.; Tischler, J..; laser, E. R.; avidson, A.; Campbell, P. M.; askill,. K., Comparison of Epitaxial raphene on Si-face and C-face 4H-SiC Formed by Ultrahigh Vacuum and RF Furnace Production. Nano Lett. 2009, 9, (7), VanMil, B. L.; Myers-Ward, R. L.; Tedesco, J. L.; Eddy, C. R.; Jernigan,..; Culbertson, J. C.; Campbell, P. M.; McCrate, J. M.; Kitt, S. A.; askill,. K., raphene formation on SiC Substrates. Mater. Sci. Forum 2009, , Tedesco, J. L.; vanmil, B. L.; Myers-Ward, R. L.; McCrate, J. M.; Kitt, S. A.; Campbell, P. M.; Jernigan,..; Culbertson, J. C.; Eddy, C. R.; askill,. K., Appl. Phys. Lett. 2009, in press. 18. Lee,. S.; Riedl, C.; Krauss, B.; von Klitzing, K.; Starke, U.; Smet, J. H., Raman Spectra of Epitaxial raphene on SiC and of Epitaxial raphene Transferred to SiO2. Nano Lett. 2008, 8, (12), Shivaraman, S.; Chandrashekhar, M.; Boeckl, J. J.; Spencer, M.., Thickness Estimation of Epitaxial raphene on SiC using Attenuation of Substrate Raman Intensity. J. Electron. Mater. 2009, 38, (6), Rasband, W. ImageJ, 1.38x; National Institutes of Health: Tuinstra, F.; Koenig, J. L., Raman Spectrum of raphite. J. Chem. Phys. 1970, 53, (3), Mas-Torrent, M.; Rovira, C., Novel small molecules for organic field-effect transistors: towards processability and high performance. Chem. Soc. Rev. 2008, 37, (4), Sun,.; Wu, Z.-K.; ivin, C.; Li, X.; Berger, C.; e Heer, W. A.; First, P. N.; Norris, T. B., Ultrafast Relaxation of Excited irac Fermions in Epitaxial raphene Using Optical ifferential Transmission Spectroscopy. Phys. Rev. Lett. 2008, 101, (15), awlaty, J. M.; Shivaraman, S.; Chandrashekhar, M.; Rana, F.; Spencer, M.., Measurement of ultrafast carrier dynamics in epitaxial graphene. Appl. Phys. Lett. 2008, 92, (4), Fang, T.; Konar, A.; Xing, H. L.; Jena,., Carrier statistics and quantum capacitance of graphene sheets and ribbons. Appl. Phys. Lett. 2007, 91, (9),

8 Supporting Information A schematic detailing the transfer procedure is presented in Supplemental Fig Pretreat the handle substrate using a standard chemical clean (if necessary) and an oxygen plasma for improved surface bonding to graphene transfer. 2. The thermal release tape is placed on the graphene surface, forming the graphene/tape stack. 3. The graphene/tape stack is placed within the bonding apparatus and a defined force is applied to the stack. 4. The stack is removed from the bonding apparatus and the tape, along with the majority of the E layers, are peeled from the SiC substrate. 5. The E bonded to the tape is placed on the handle substrate (e.g. SiO 2 on Si) and is placed in the bonding apparatus where a similar force is applied to the E on handle substrate stack. 6. The E on handle substrate stack is removed and is heated on a hot plate maintained at a temperature greater than the release temperature of the thermal release tape used. The tape, after losing its adhesive properties is then removed from the sample surface. A 250 o C hotplate anneal in atmosphere and solvent clean may be used to improve graphene/substrate adhesion and remove the tape residue, respectively. SYNOPSIS TOC 8